Primarily because of their use in optical communications systems, the fabrication of optical fibers has been a subject of intensive research and development. Optical fibers are typically made in a continuous process which involves drawing the thin glass strand or fiber from a partially molten glass preform, and thereafter coating it with a polymer to increase its structural strength. The U.S. Patent of Andrejco et al., 4,450,333, describes in detail a furnace of the type that can be used for heating a glass preform to permit a fiber to be drawn.
Optical fibers as presently made are still much more fragile than metal conductors, and they are subject to increased light transmission loss due to hydrogen contamination. Contamination is particularly a problem if the optical fiber is used as part of an underwater cable because permeation by water or OH radicals may result in an undesired reaction with the glass (silicon dioxide) optical fiber and because, in that environment, fiber replacement or repair is comparatively inconvenient and expensive. The copending application of DiMarcello et al., Ser. No. 098,253, filed Sept. 18, 1987, and assigned to Bell Telephone Laboratories, Inc. (hereby incorporated herein by reference ), describes a method for hermetically sealing the optical fiber by coating it with a thin carbon film. The fiber is coated by exposing the hot fiber from the furnace to an atmosphere of, for example, acetylene and other gases, which results in a carbonaceous coating having a form which is particularly reliable and effective in protecting the glass fiber from contamination. In particular, with the method described, the coating results in a cross-linked carbon network which increases the strength of the fiber as well as providing protection.
One problem with using the DiMarcello et al. method is the difficulty of monitoring the fiber during production to assure that a carbon coating of the proper thickness is being applied. Since the coating is only 500 to 1000 angstroms thick, normal mechanical methods of monitoring coating thickness cannot be used. Presently, coating thickness is determined by measuring the electrical conductivity along samples of the optical fiber. Since both the glass fiber and the polymer coating are non-conducting, electrical conductivity along the sample length is a function of the carbon thickness. This is normally a destructive testing method, in that it requires separation of the fiber sample from the fiber under production. There is thus a need for a simple and accurate method of determining carbon coating thicknesses on optical fibers, and preferably a method that is nondestructive to the optical fiber. Further, there is a need for such measurement in a way that would provide feedback during the production for adjusting coating thickness during a continuous production process to assure that it remains within prescribed limits.